The old US Divers Aqua-Lung Aquarius from the 1970s also makes a good pony bottle reg. They are extremely simple (just one moving part), rugged and reliable. Bryan at Vintage Double Hose sells service kits for them and, best of all, they are super cheap to buy on Ebay. The Aquarius' performance is very similar, if not identical, to the Calypso. In fact, the current Calypso is pretty much an evolution of the older Aquarius.
The above said, my youngest son uses a Calypso as his primary dive reg and he likes it so much that he won't even consider getting another regulator.
---------- Post added February 3rd, 2013 at 02:36 PM ----------
After I wrote the above, I went on Ebay and found one of the relatively rare four port Aquarius first stages (from the '80s) in very good condition for $40. Of course I bought it. I love the old Aquarius and I like to keep a few around. They are so simple and easy to service, they make an excellent reg for divers who want to begin servicing their own regs. The Aquarius was designed to be serviced in the field with no special tools or equipment. I have serviced them with nothing more than a Swiss Army knife and an adjustable wrench! Except for the HP seat and sintered filter, all there is to replace are easy to find O-rings.